1. Field of the Invention
The invention refers to a method and a plant for the cooling and selective separation of chlorides and fluorides out of hot mixtures of gases generated by gasifying solid or liquid fuels or by extracting gas from solid fuels.
2. Description of the Prior Art
The gas which occurs in the case of the extraction of gas from coal or the gasification of it contains besides the acid constituents CO.sub.2, H.sub.2 S, HCN, inter alia HCl, HF and also NH.sub.3, the concentration of which is dependent upon the coal used and the conditions of the process.
In the case of the preparation of gas for use as fuel or for synthesis, essentially physical or chemical wet washing processes are applied in order to remove the acid constituents. The prerequisite for the application of wet washing processes for desulphurizing and the removal of carbon dioxide is that the chlorides and fluorides are removed beforehand in order to keep the concentration of chlorides and fluorides in the washing circuit at justifiable values without discharges of washing liquor. The discharge of washing liquor must be avoided since the disposal of the amounts discharged, apart from the costs for the introduction of fresh lyes, can only be performed at considerable expense without burdening the environment. Further, the wet processes for desulphurization and the removal of carbon dioxide demands prior cooling down of the gases before entry to 30.degree. to 100.degree. C. and HCl in the gaseous phase form solid NH.sub.4 Cl which is deposited upon the cooler surfaces and from time to time must be removed. Continuous operation with these heat-exchangers is therefore not possible.
Attempts have therefore already been made to cool the gas in washers directly by evaporation of H.sub.2 O and in so doing at the same time to separate the chlorides and fluorides. For this purpose fresh water is fed into a washing circuit. The NH.sub.3 present in the gas combines with the acid constituents of the gas, preferably with HCl and HF.
Besides the acid constituents the gas occuring the case of the extraction of gas from coal or gasification of it, also contains steam. In particular in the case of the pressure processes the steam contained in the gas condenses in the case of cooling of the gas down to, for example, 40.degree. C. almost completely. The amount of condensate is then the amount of effluent which has to be discharged with the separated chlorides and fluorides. But in future effluents containing chlorides and fluorides must no longer be released into drainage ditches. But the removal of chlorides and fluorides from the effluent is possible practically only by evaporation which, however, demands considerable amounts of power.
In the case of direct cooling by evaporation of water out of drops of solution, particularly in the case of a high concentration of ammonium chloride and the temperatures which are here being striven for, ammonium chloride aerosols come into existence. These aerosols originating from the liquid phase can only be separated again with difficulty, for example, in a washer having a high pressure drop or in wet electrostatic filters. In the case of the direct cooling by a cooler in the liquid circuit down to below the adiabatic saturation temperature, which demands very high liquid/gas ratios, ammonia, hydrochloric acid and hydrofluoric acid are preferably separated by absorption into the liquid in the circuit. But the absorption effect worsens with rising temperature in the liquid circuit.